When sewing machines are operated on a continuous basis at high speed for forming the hems of sheets and similar long run applications, it is highly desirable to immediately detect any breakage or depletion of thread in the system. Otherwise, the system will continue to operate without thread extending to the needle and the work product will be improperly formed. It is difficult and time consuming to reposition the work product in the proper folded configuration at the sewing machine needle if the work product has continued to move some distance after the breakage of thread.
Several stop motion systems have been developed in the past to detect the breakage of the needle thread and to terminate the operation of the sewing machine motor in response to such detection. For example, U.S. Pat. Nos. 3,587,497, and 4,186,672 disclose stop motion systems.
One of the prior art stop motion systems used for the detection of needle thread breakage operates by permitting the check spring that is reciprocated in response to varying thread tension to make electrical contact with a contact block when the thread is relaxed. Because the check spring is moved on each cycle of the sewing needle as the thread is tightened and then relaxed by the takeup arm, the circuit to the contact block made by the check spring is opened during each cycle. Thus, by detecting the opening and closing of the circuit made between the check spring and its contact block, a continuing signal is provided as long as the needle thread is properly fed to the sewing needle and is stitched into the work product.
One of the problems of the above-noted check spring and contact block arrangement is that over prolonged use the lint, dust and oil in the normal environment of the sewing machine accumulates at and around the contact block and/or check spring and increases the electrical resistance between these elements, so that a proper signal might not be generated in response to the movement or the absence of movement of the needle thread in the sewing machine. Further, there are times when the needle thread breaks at the needle but the thread continues to move, not through the needle but through the presser foot with the thread being carried by the work product in the sewing direction, which tends to pull the thread continuously with the work product. This is likely to cause the check spring to continue to oscillate against its contact block in response to the reciprocation of the takeup arm and generate the same signal as when the thread is not broken and is properly moving through the eye of the sewing needle, so that the detector system does not properly function to detect the broken thread.